For your purposes, I would recommend: 20 or 25 gallon, oiled, belt drive compressor, as big a motor as well run on whatever electric feed you have. Make sure it'll pump up to 125PSI or more.
For the impact gun, replace the tank's outfeed quick release with a High Flow like a Milton V, and the hose end to match. (Doing just one or the other may work, but will accomplish nothing.) You'll run the tank down faster, but your impact will hit harder, and after doing all the lug nuts on one side, by the time you move to the other side of the vehicle, it'll catch up.
I am speaking as someone who successfully operated an 450 ft/lb IR impact off of a 10 gallon tank / 1/3HP for several years, before upgrading to 60 gallons / 135PSI / 13.5CFM. With the 10 gallon, by simply planning some activity between vehicle sides, I could stay productive during the recharge cycle.